1. Field of the Invention
This invention relates to a process for curing unsaturated polyester resins.
2. Description of the Prior Art
Unsaturated polyester resins are widely used in the field of composite materials such as, for example, castings, paintings and fiber glass products.
As the use of unsaturated polyester resins has thus expanded and unsaturated polyester resins are mass-produced, the producing process, curing method, curing conditions, characteristics and cost of curing moldings of unsaturated polyester resins are studied with the result that the cycle-up of molding, simplification of working process, reduction of curing cost and fast curing of the curing agent thereof are achieved.
In order to promote the hardening of an unsaturated polyester resin when the unsaturated polyester resin is cured by using an organic peroxide, it is most common to employ a curing accelerator for promoting the radical decomposition of the organic peroxide, and the representative one is a process which uses a Redox catalyst.
One exemplification of the process is that using the combination of a ketone peroxide and a metallic soap, particularly cobalt salt which are generally used in the hardening of unsaturated polyester resins at room temperature.
However, this process has such disadvantages that the curing time of the unsaturated polyester resin is long, the cured material is colored pink by the cobalt salt, and it is difficult to completely harden the unsaturated polyester resin in a cold area.
In order to overcome these disadvantages of the conventional process, various processes have been proposed for promoting the curing of unsaturated polyester resins by adding a third material to the Ketone peroxide and cobalt salt as an auxiliary curing agent. Of these proposed processes, there are processes which use a .beta.-diketone of Keto-enol tautomer (See U.S. Pat. No. 3584076) and cyclopentane derivatives of .beta.-dicarbonyl compounds (See British Pat. No. 1,293,209 ) as the third material.
However, these processes have an indispensable requirement, namely, that a metallic salt such as a cobalt salt, must be used, and therefore, not only is there imparted to the product the peculiar color of the metallic salt, but also a chelate compound of the third material and the metallic salt is formed with the result being that the chelate compound imparts its peculiar color such as, for example, green, to the product to cause it to have an uneven color. In addition, since a gloomy feeling is imparted to a decorative plate when these processes for curing unsaturated polyester resin are employed to produce the decorative plate, they are not practical.
In addition, as the process for curing unsaturated polyester resins using the third material has complicated the working steps and has increased the curing cost, it is not practical except when high speed curing is particularly required.
There is also disclosed a process for curing unsaturated polyester resins using an organic peroxide and a tertiary amine. However, the diacyl peroxide, an organic peroxide commonly used in this process, is extremely sensitive to friction and shock and is also easily exploded and therefore is very dangerous to use. Accordingly, this process has disadvantages in that it involves complicated steps for the treatment of the dangerous compound, for instance, a step which changes the dangerous compound into a paste-like form using a diluent, in that the inhibition action due to air is great when the resin is cured, in that the product is strongly colored orange-yellow and, in addition, tends to discolor due to light, heat, etc.